This invention relates to coating formulations and a method, useful in microelectronics applications, for isolating and protecting fine-pitch, electrically conducting circuit interconnects, and related structures. More particularly the invention provides coating materials for application to conductive elements using an electrophoretic deposition technique. The coatings are non-contaminating, low-outgassing, and protective, having high resistivity and low dielectric constant. Optionally, the coatings provide negative image bearing layers after exposure to radiation patterns of suitable wavelength, followed by development with mild aqueous acid solutions.
Modern society relies upon the trouble-free conveniences provided by electrical and electronic devices. Since the earliest recognition that useful devices could be developed by combining electrical circuits, circuit combinations have become more complex, and the resulting devices more sophisticated in their capabilities. Effective circuit performance relies upon electrical current isolation within a particular circuit with no possibility of current leakage into a neighboring circuit. Any unintended current transfer between circuits of a multi-circuit, multi-function electrical device will ultimately cause an inconvenient malfunction of the device.
Isolation or insulation of circuits from each other represents an increasing challenge with the continuing emphasis on more complex printed circuit designs and increased functionality for electrical devices, especially miniature electronic devices. Progress in electrical device design has caused a transition from the interconnection of discrete electrical components, using pre-insulated wiring structures, to interconnection, with modern printed circuits, using conductive traces only microns wide. Protection and isolation of such narrow traces, from each other, demands materials that may be precisely placed over the elongate current carrying traces while leaving tiny contact points exposed for electrical connection to other circuits that form part of a particular device. For a significant period of time it was possible to essentially cover the printed circuit with a protective coating, leaving voids in the coating corresponding to the needed points of contact. More recently, however, the introduction of flexible printed circuits and multi-layer printed circuits has led to the need for coatings and processes capable of high precision in protective cover formation and placement. High precision techniques provide a cover-layer with essentially just sufficient insulation to protect a conductive trace without straying into other portions of a printed circuit substrate. Such coatings tend to be very thin and subject to attack by, e.g. solvents, moisture, or other potentially damaging environments. For this reason, precision coating of printed circuits requires both insulative and environmental protection for electrical conductors.
A variety of coating methods exists for applying coatings, covercoats and the like as protective, insulating coatings to printed circuit patterns. The term covercoat refers to a dielectric coating, over the printed circuit basestock, applied after the conductive circuit pattern has been fabricated. The covercoat serves to protect the conductive traces from moisture, contamination and damage. Conventional coating methods include screen printing and application of continuous layers by methods such as knife coating, spin coating, extrusion coating, dip coating, curtain coating, and spray coating. Application of continuous coatings covers not only the leads but also the area in between the leads. This condition has several disadvantages when found in intricately structured printed circuits. For example, differences in expansion coefficients between a continuous cover-coat and a flexible printed circuit substrate may introduce stresses that cause the circuit to adopt an inconvenient curl-set. Segmentation of a cover-coat, into separate coated areas, is less likely to be subject to this condition.
Selective deposition processes, such as electrophoretic deposition, also known as xe2x80x9ce-coat,xe2x80x9d may achieve coating separation and precise positioning (details of this process may be found in the xe2x80x9cHandbook of Electropainting Technologyxe2x80x9d by W. Machu, Electrochemical Publication Limited, 1978). Application of electrophoretic deposition techniques began at least three decades ago for painting automobiles and appliances. Electrophoretic deposition involves precise distribution of a layer of charged droplets over a conducting surface that represents an electrode of an electrolytic cell operating under direct current potential. Charged droplets migrate towards an oppositely charged electrode to be deposited thereon. Droplet deposition and layer formation may occur at either an anode or a cathode. Preferably the droplets are positively charged for deposition on a cathodic surface. Cathodic coatings do not suffer the oxidative corrosive processes associated with anodic deposition. Also, electrophoretic deposition of water-based compositions produces essentially void free and substantially non-polluting coatings.
Compared to conventional coating processes, such as screen printing, electrophoretic deposition selectively places a protective layer only on conductive portions of the printed circuit. Use of electrophoretic deposition should produce individually encapsulated conductors, whereas conventional techniques coat the entire printed circuit. Selective deposition also offers other advantages, such as the production of lighter weight circuits which is important for hard disk drive (HDD) flexible circuits applications. Japanese published application JP 2000004072 describes a method for selectively exposing a connective portion of a circuit structure on the rigid metal surface of a magnetic head suspension arm. This reference uses electrodeposition of resins selected from polyimide or acrylic or epoxy resins to mask parts of a circuit structure during application of photoresist material. Imaging of the photoresist identifies, after development, parts of the circuit from which the electrodeposited covering will be etched to expose bare metal for contact points. There is no evidence to show that electrodeposited coatings are required to protect against moisture and contaminants. Also there is no need that the electrodeposited coatings are flexible coatings since they are substantially prevented from flexing by the support of the underlying aluminum suspension arm.
Selected electrical connector contacts on a flexible support may be electrophoretically coated with a blend of a polyethylene ionomer emulsion and an epoxy ester polymer emulsion, according to U.S. Pat. No. 3,892,646. Coating deposits may be thermally coalesced but are non-crosslinking. Electroconductive adhesives may be electrodeposited over fine circuit patterns to facilitate connection to other substrates preferably having electroconductive patterns, as described in U.S. Pat. Nos. 4,676,845 and 4,844,784.
The use of electrophoretic deposition is known for coating printed circuits with photoresists. U.S. Pat. Nos. 4,845,012; 4,877,818; 5,055,164; 5,607,818; 5,384,229; 5,959,859; and 5,439,774 contain reference to the technique. Other U.S. Pat. Nos. 4,592,816 and 5,181,984 describe epoxy/acrylate compositions for electrophoretic deposition of solder mask/covercoat systems. Photoresist and solder mask materials are typically photosensitive and developable to a patterned polymer covering selected (imaged) portions of the printed circuit. This provides evidence of photoimageable coatings, formed by electrophoretic deposition. Additionally, U.S. Pat. No. 4,832,808 teaches electrophoretic deposition of coatings of piperazine-containing polyimides. However, such coatings possess neither photosensitivity nor solubilization in aqueous acid developers.
The effective use of electrophoretically deposited, optionally photoimageable, coatings may depend upon the image resolution attainable with such systems. Printed circuits of increasing density require the use of photoresists of increasing image resolution. Image resolution depends upon radiation scattering within photosensitive layers and the variation of image characteristics, i.e. resolution, related to developers and development processes.
Polyimide-containing formulations provide potentially useful materials for photoimageable coatings produced by electrophoretic deposition. They also have the thermal and dielectric properties suitable for protecting and insulating electrical current carrying conductors. Image development of polyimide coatings, after exposure to an image pattern, may involve non-aqueous, solvent-based developers or aqueous-based developers. The use of solvent-based development systems applies to photoimageable polyimides that may use a benzophenone moiety as a built-in photo-crosslinker. U.S. Pat. Nos. 4,629,685; 4,656,116; 4,841,233; 4,914,182; 4,925,912; 5,501,941; 5,504,830; 5,532,110; and 5,599,655; and European Patent No. EP 0456463 A2 provide evidence of autosensitized polyimides.
Little attention has been given to an area of growing importance wherein a flexible interconnect carries signals to a magnetic transducer, located on a slider supported by a load beam within a hard disk drive (HDD). U.S. Pat. No. 6,046,886 refers to problems with the use of cover dielectric layers that may be used to insulate copper traces of a flexible interconnect. This reference further emphasizes the benefits of eliminating cover coat layers to avoid chemical incompatibility from outgassing and contaminating extractables such as silicones and ionic contaminants. Another problem with the application of cover coat layers to flexible circuits is the potential mismatch of thermal expansion coefficients between a cover coat and a flexible substrate used to support conductive circuit traces. Due to the continuous nature of cover coat layers, any mismatch of thermal expansion could cause the flexible circuit interconnect to develop an undesirable curl-set.
Considering the disadvantages of previously discussed cover coated flexible interconnects for hard disk drive applications there is a need for a coating to protect conductive traces using a process that avoids problems of curl and chemical incompatibility.
The present invention provides polymeric coatings applied to conducting structures, such as thin-film printed circuits, from emulsion or solution formulations using electrophoretic deposition preferably cathodic electrophoretic deposition techniques, also referred to herein as cataphoretic deposition. A coated thin film supported circuit, suitable as a HDD interconnect according to the present invention, is electrically insulated, exhibits minimal curl, and is minimally outgassing. It has flexibility similar to the thin-film supporting layer, may be imageable and may be formulated and cured such that electrical connections can be made through the coating by appropriate soldering techniques. A post-process to image holes through the coating may comprise laser ablation. Optionally, such coatings may function as image recording materials that respond to exposure to a pattern of suitable radiation. An image, formed in a coating according to the present invention, may be revealed using an acidified aqueous developer.
Coating compositions according to the present invention may comprise a variety of polymeric materials including polyimides, epoxy, acrylics, etc., especially those using epoxy chemistry. Electrophoretic deposition techniques require water-based coating formulations that are relatively benign environmentally. Suitable emulsion formulations include ionic species that are preferably positively charged to allow precipitation at cathodic surfaces represented by the conductive traces of a thin-film printed circuit. After deposition and curing, appropriately formulated coatings, based on epoxy or epoxy/acrylate chemistry have glass transition temperature (Tg) values in the range of 75xc2x0 C. to 200xc2x0 C. and preferably between about 100xc2x0 C. and about 160xc2x0 C.
Versatile coating compositions according to the present invention may be formulated and manufactured to satisfy a number of different applications. For example, coatings may be deposited so that they do not cure, or only partially cure after deposition and coalescence or after coalescence and UV exposure. Such coatings may be penetrated relatively easily to form electrical through-connection, such as by soldering. Alternatively, coatings may be formulated to have latent curing characteristics. A thermally cured material provides one type of latent curable coating composition that may be deposited in an uncured state before heating to a final cured condition. As needed, an electrical connection may be provided to underlying conductive circuit traces by soldering through the latent curable deposit, which may then be cured by exposure to a source of thermal radiation.
Latent curing may also be achieved using, e.g. a photo-crosslinkable coating composition exposed to a suitable form of radiant energy, such as ultraviolet radiation. The use of latent photo-cure provides not only the benefit of soldering through an uncured coating, as discussed previously, but also allows selective placement of protective insulating material following photo-curing and development according to imagewise exposure to appropriate radiant energy. This could provide xe2x80x9cconnection padsxe2x80x9d positioned appropriately for soldering in the normal sense. Control of cross-link density may provide coatings suitable for forming through-soldered connections even after photo-cure.
Another possible combination of latent curing characteristics involves coating compositions that have been formulated to cure in response to both radiant energy and heat. This provides a photo-imageable and developable coating that has optimum cured resin performance upon heating.
Methods for cathodic deposition of coatings include batch and continuous processes. An electrophoretic deposition cell facilitates batch processing of individual circuits using a container for a water-based coating formulation that is stirred to maintain uniformity. The cell has an anode, in the form of a stainless steel counter-electrode, and a cathode, both being connected to a regulated DC power supply. Preferably, the cathode comprises a thin film circuit designed with interconnection or xe2x80x9cbussingxe2x80x9d of metal traces to facilitate deposition of material on all portions of the circuit that require coating. Voltage applied to the cell, after lowering the anode and cathode into the coating formulation, causes coating to deposit on electrically connected portions of the thin film circuit. It is not necessary to regulate the current, which initially peaks to a certain value before dropping due to the insulating effect of the depositing coating. Voltage applied for times ranging from a few seconds to a few minutes may be as low as 5 volts or less, or as high as 100 volts. Following deposition and rinsing with deionized water, the particulate deposit may be coalesced to a smooth coating by heating for several minutes at elevated temperature. The coating can then be subjected to any of the cure methods described above.
Alternatively, the coating may be deposited in an in-line, continuous process that uses a insulating film support web that has multiple printed circuits formed over its surface. In this case, appropriately xe2x80x9cbussedxe2x80x9d circuits maintain electrical continuity along the web, between individual circuit elements. The coating apparatus allows threading of the circuit supporting web from an unwind station through an electrophoretic deposition cell to a wind-up station. Printed circuits according to the present invention typically include a polyimide thin film support for electrically insulated circuit traces. Preferably the thin film is Kapton E and the circuit traces may be formed using copper alone or copper that has been electrocoated with gold. As the moving web passes through the deposition cell an electrical current carrying roller makes connection with the circuit metal traces to apply a voltage causing coating to deposit on electrically connected conductive traces. Other rollers in the web guidance system are suitably insulated to prevent inadvertent coating deposition. There is no need to control applied current since the thickness of the insulating coating composition is current limiting as described previously. After in-line rinsing, the coated circuits may be coalesced at elevated temperature by exposure to an infrared heater or contact of the backside of the web with a hot surface, typically in the form of a hot can. The coalescence step may be delayed since the deposited coating is inherently a solid and suitable for wind-up into a roll for future processing. It is conceivable to add additional processing stations to process thin film circuits, in web form, for thermal curing, photo-curing and selective development of imaged sections of deposited coatings.
One use of photoimageable polymers, including photosensitive epoxy-based polymers, is the precise electrophoretic deposition of protective, electrically insulating coatings over conductive parts of a printed circuit pattern, followed by imagewise exposure and development. The process of development uses aqueous acid or aqueous acid/coalescing solvent mixtures to remove the coating from those parts of the circuit that provide points of connection to other circuits or electrical devices. Acidified aqueous developers offer advantages over solvent and aqueous alkaline developers by preventing problems of copper corrosion and copper oxide formation.
More particularly the present invention provides a composition for electrophoretic deposition of a protective coating. The composition comprises a cationic resin emulsion and a curative mixed with the cationic resin emulsion. The composition after electrophoretic deposition and curing provides the protective coating that has a concentration of extractable ionic contaminants less than about 200 nanograms/cm2; and a concentration of labile (outgassing) components less than about 36,000 nanograms/cm2.
The present invention further includes an article in the form of a flexible printed circuit, used as an interconnect comprising a film substrate having a thickness from about 0.01 mm to about 0.25 mm. A plurality of conductive traces is adjacent to a surface of the film substrate to receive an insulating coating deposit on the plurality of conductive traces by electrodeposition techniques. The insulating coating comprises a cured polymer composition having a concentration of extractable ionic contaminants less than about 200 nanograms/cm2; and a concentration of labile (outgassing) components less than about 36,000 nanograms/cm2. Also the flexible printed circuit has a bend radius below 3 mm without damage to the insulating coating.
A method for forming an insulating coating on conductors of a flexible circuit, according to the present invention, comprises the steps of initially providing a flexible circuit including at least one conductor. The conductor is available for connection to a DC power supply such that the conductor becomes a negatively charged conductor. The negatively charged conductor is immersed in a composition comprising a cationic resin emulsion mixed with a curative. An electric current is then passed through the negatively charged conductor to cause electrophoretic deposition of the composition on the surface of the conductor. Curing of the deposited composition provides the insulating coating that has a concentration of extractable ionic contaminants less than about 200 nanograms/cm2; and a concentration of labile (outgassing) components less than about 36,000 nanograms/cm2. These properties satisfy the requirements of electronics-grade cleanliness.
Electrophoretic deposition techniques allow relatively precise placement of material on charged surfaces included in an electrolytic cell, operated by direct current. The charged surfaces could include suitably connected printed circuits to induce material placement on individual metal traces of the circuitry. Using electrophoretic deposition techniques, deposition of material occurs predominantly on conductive surfaces. This facilitates the coating of unsupported leads and relatively inaccessible portions of a printed circuit such as conductive traces disposed within the structure of a multilayer circuit. Traditional coating methods do not provide desirable protection for such features. In addition, precision coating via electrophoretic deposition techniques uses less material than traditional coating methods thereby providing beneficial cost savings and waste reduction. The selective placing of electrophoretically deposited films provides an added advantage, for coating flexible printed circuits, compared to blanketing covercoat layers produced with conventional coating methods. Regardless of differences in coefficient of thermal expansion, selectively deposited coatings cannot exert a force to distort the general shape of the flexible substrate material. Flexible circuits, coated using electrophoretic deposition, are lighter and less likely to exhibit cure-stress-induced curl after processing. Lower circuit weight is important for certain applications, such as interconnects for hard disk drives.
For clarification, the following definitions provide the meaning of terms that may be used throughout this specification.
The term xe2x80x9ccovercoatxe2x80x9d refers to a substantially continuous dielectric coating, over the basestock, applied after the conductive pattern has been fabricated. The basestock may be a conventional printed circuit substrate, including flexible polyimide sheet, used as a support for etched metal patterns, particularly those formed by etching copper.
The terms xe2x80x9cepoxy-based polymerxe2x80x9d or xe2x80x9cpolyepoxy-based polymerxe2x80x9d or xe2x80x9cpolyepoxide-based polymerxe2x80x9d may be used interchangeably to refer to the product of a reaction involving monomers and oligomers having reactive epoxy substituents.
Use of the term xe2x80x9cbis-maleimidesxe2x80x9d herein refers in general to the reaction product of maleic anhydride with an aliphatic diamine, or an aromatic diamine, or an alicyclic diamine or any combination thereof.
The term xe2x80x9ccurrent densityxe2x80x9d means the amount of current flowing through a substrate, per unit area, perpendicular to the direction of current flow.
The term xe2x80x9ce-coatxe2x80x9d is synonymous with electrophoretic deposition and may refer herein to a coating, and technique for electrophoretically depositing such a coating.
The term xe2x80x9cemulsionxe2x80x9d refers to polymer containing fluids used for electrophoretic deposition of protective coatings on electrically conducting surfaces.
Use of xe2x80x9celectronics-grade cleanlinessxe2x80x9d herein means that selectively-coated, thin-film supported circuits according to the present invention satisfy electronics industry requirements to limit amounts of labile or fugitive components to low levels of outgassing and extractable contaminants and absence of tin and silicone species.
The term xe2x80x9cthrough-solderingxe2x80x9d refers to the facility for making an electrical connection to a conductive circuit trace through a layer of a protective coating composition. Depending upon the final properties of the coating, a through-soldered connection may be formed before or after curing of a coating, preferably an electrophoretically deposited coating.
Use of the term xe2x80x9claser ablationxe2x80x9d refers to a means for providing access to bare conductors of a printed circuit using laser energy to produce openings in a coating over the conductors. The laser ablation method provides holes as access channels for electrical connection of underlying conductors to electronic components and devices, e.g. by soldering.
The use of the term xe2x80x9cselectivexe2x80x9d reflects substantial restriction of protective coating to only those portions of a printed circuit that are electrically conductive. Advantages of selective coating include weight reduction, the use of less material and significantly less induced curl than conventional coating methods.
The term xe2x80x9cunsupported leadxe2x80x9d means a conductive trace or lead positioned adjacent to but not necessarily supported by the insulating substrate of a flexible circuit. An example of an unsupported lead is one that spans a void in a substrate or extends over the edge of a substrate and thereby exists in an unsupported condition.
Conductive leads or traces xe2x80x9cadjacentxe2x80x9d to an insulating support or substrate may include those in contact with the support or those in the vicinity of the support without contacting the support, or both.
The term xe2x80x9caqueous acid soluble polymerxe2x80x9d refers to a polymer that is at least partially soluble in aqueous acid solutions.
The term xe2x80x9caqueous acid developable polymerxe2x80x9d refers to a photoimageable, aqueous acid soluble polymer crosslinked by exposure to suitable radiation so that crosslinked material no longer dissolves in dilute aqueous acid. This allows dissolution of unexposed material to leave an insoluble pattern of crosslinked material corresponding to the pattern of radiation used for exposure.
The term xe2x80x9ccoalescing solventxe2x80x9d refers to a class of solvents that are water soluble as well as oil soluble. These solvents, e.g. 2-butoxyethanol (butyl cellosolve), facilitate the coalescence of the coating after deposition.
The term xe2x80x9cresin solventxe2x80x9d refers to a class of solvents that are substantially water insoluble but in which the cataphoretic resin is soluble. Solvents such as xylene, mixed xylenes, toluene, iso-amyl ketone, methyl iso-butyl ketone and the like facilitate the processing of cataphoretic resins.
Concentrations expressed as percentages (%) signify weight percent (wt. %) unless otherwise indicated.